The fuel gas powered heat producing device is among the most common of appliances. A fuel gas, typically natural gas, propane or butane is ignited and burns within a combustion chamber. The resultant heat is used to heat water or air, or for a wide variety of industrial process applications. Millions of such devices are found throughout the world. Problems associated with the use of these devices include the generation of toxic combustion products if the fuel is improperly burned. Since the fuel gas is combustible, any leakage brings with it the hazards of fire or explosion.
In most places, safety systems to monitor the combustion process and to terminate the flow of fuel gas if a fault occurs are mandated by law. The vast majority of these heating devices contain a pilot burner which is in continuous operation. This "standing pilot" provides a source of ignition whenever fuel gas enters the main burner. A thermoelectric device, typically a thermocouple, is placed in the pilot flame and the resultant electric current produced operates an electromagnet in the fuel valve which holds it open and allows fuel gas to flow to the main burner. If the pilot flame fails for any reason, the thermocouple slowly cools and stops producing electric power, releasing the electromagnet which causes the valve to close.
Thus, the standing pilot thermoelectric system is totally self-powered. The main disadvantage of this system is its slow response. As much as 3 minutes may pass after the pilot flame failure before closure of the valve takes place. For larger burners, any delay in operation of the safety circuit would mean that a considerable volume of unburned fuel gas could flow before the valve closes. This can lead to fire or explosion. For this reason, larger burners utilize electronic flame sensors which have much faster response times. One example of a widely used electronic device is the flame rectifier. These devices, although capable of shutting off the flow of fuel within four seconds, require enough electric power that connection to the A.C. power line is required. As a consequence of the A.C. power requirement, to be safe, the control must be designed to shut off the appliance in the event of A.C. line failure.
Another device for this purpose is the subject of U.S. patent application Ser. No. 659,074 filed Oct. 5, 1984 which is a continuation in part of U.S. patent application No. 517,699 filed Jul. 25, 1983. The subject matter of these applications is hereby incorporated by reference. It teaches the use of an emissive element placed within the pilot flame and a photovoltaic cell to convert the resultant radiation to electric power which is used to operate the safety electromagnet in the gas valve. Its speed of operation is faster than the flame rectifier yet, like the much slower thermocouple driven systems, it is entirely self-powered.
Many heating devices have automatic controls for the regulation of temperature. In the simplest of these devices, such as the type found on the typical storage type water heater, the fuel valve is controlled by a thermomechanical operator which senses the tank temperature. The contraction and expansion of the operator controls the valve directly. Such controls are simple and self-powered but allow temperature measurement at only one point in the tank. Because of consideration of the location of the fuel supply pipe, the control is usually placed in the vicinity of the burner, near the bottom of the tank. As a consequence of the low placement of a single temperature measurement point and under certain patterns of use, an overheating condition called stacking may occur. If the temperature control were done with electronic devices, the temperature at multiple points in the tank could be sensed, thus eliminating this problem.
In other types of devices, such as space heaters, remote sensing of the temperature is necessary, since the sensing point may be some distance from the burner. Some form of electronic control is usually used. Although thermopile operated valves are capable of operating with remote thermostats, they require standing pilots in order to provide the power for valve operation. Some heaters utilize mechanical or hydraulic thermostats which directly control the valve. Most other systems employ electric thermostats in conjunction with valves powered by the A.C. line.
In recent years, as the energy crisis produced a dramatic rise in the price of fuel gas, many gas appliances were redesigned in order to increase their operating efficiency. Standing pilots were replaced with intermittently operating ones with electrically operated ignitors. Thermostats were replaced with clock controlled automatic setback types. Automatically operated vent dampers were added to trap warm air within the combustion area when the flame was not present. Unfortunately, all these devices require the appliance to be connected to the A.C. power line which meant that A.C. power must be available in the vicinity of the appliance, and, even though ample fuel gas is available, the heating system will not work in the absence of A.C. power. This represents a real hazard for residential heating in winter, since A.C. line failure due to severe weather conditions does occur.
There is a need for a system for use with combustion appliances which provides the efficiency improvements described above but would derive all power needed from the flame itself, without any requirement for outside electric power. This invention pertains to just such a system for powering the fuel control valve, thermostat, electric ignitor, flue damper and, if desired, an air circulating fan, with all power being derived entirely from the flame.
A basis for this system is the invention as described in the patent application mentioned above. An emissive element is placed within a flame, and the resultant radiation strikes a photovoltaic cell which produces electric power. The emissive element may be either a metallic screen or mesh which radiates in the manner of a black body in the flame, or it may be a quantum stimulated emitter which radiates at specific wavelengths. The photovoltaic cell may be an array of photovoltaic cells made of Silicon or Copper Indium Diselenide or other photovoltaic material. The photovoltaic cell may be designed such that a potential of several volts is generated. This may be used to recharge a rechargeable battery which may assist in operations such as valve opening and ignition which must be accomplished before the flame is present. With properly designed components, power from the battery is needed only transiently, allowing ample time for battery recharging. Thus, the invention not only provides fast flame failure detection but also acts as the power source so that the control system may be completely self powered.
It is possible to utilize thermoelectric devices for some of the functions. Self-powered ignition systems have been described by Weber, U.S. Pat. No. 3,174,535 and Ryno, U.S. Pat. No. 4,181,413. Both of these utilize thermopiles as the source of electric power. Since the electric potential generated by a thermopile is low, multiple devices are needed. Each thermopile is itself an assembly of many thermocouples. Hence, they are expensive to fabricate. Since they have a very large thermal mass, their output is considerably delayed after burner ignition, and power continues to be produced long after the flame is extinguished. Thus, they are not suitable for the dual functions of power production and flame sensing.
In order to accomplish the task of making a comprehensive selfpowered safety, ignition, and control system for combustion appliances, specially modified components must be employed. They must be arranged so as to minimize the electric power needed. In particular, the fuel valve must be of a type which needs power only to open, and once open, requires no further power from the battery. Such a design is the double latched valve herein described.
In a typical standing pilot gas valve, the first stage valve is latched in the open position by a thermocoupled powered safety magnet and an electrically powered solenoid is used to open the second stage. No means is provided to latch open the second stage, so the solenoid must be continuously energized. Considerable power is needed to keep the valve open. Valves designed for intermittent ignition service are not latched at all and therefore the operators for both stages necessarily consume electric power. Thus, all present intermittent ignition systems are powered by the A.C. line.